Description: The Antarctic Cold Reversal (ACR; 14.7 to 13 ka) phase of the last deglaciation saw a pause in the rise of atmospheric pCO2 and Antarctic temperature, contrasted with warming in the North. Mechanisms associated with interhemispheric heat transfer have been proposed to explain features of this event, but the response of marine biota and the carbon cycle are debated. The Southern Ocean is a key site of deep-water exchange with the atmosphere, hence deglacial changes in nutrient cycling, circulation, and productivity in this region may have global impact. Here we present a new perspective on the sequence of events in the deglacial Southern Ocean, that includes multi-faunal benthic assemblage (foraminifera and cold-water corals) and geochemical data (Ba/Ca, 14C, δ11B) from the Drake Passage. Our records feature anomalies during peak ACR conditions indicative of circulation, biogeochemistry, and regional ecosystem perturbations. Within this cold episode, peak abundances of thick-walled benthic foraminifera and cold-water corals are observed at shallow depths in the sub-Antarctic (~300 m), while coral populations at greater depths and further south diminished. Geochemical data indicate that habitat shifts were associated with enhanced primary productivity in the sub-Antarctic, a more stratified water column, and poorly oxygenated bottom water. These results are consistent with northward migration of primary production in response to Antarctic cooling and widespread biotic turnover across the Southern Ocean. We suggest that expanding sea ice, suppressed ventilation, and shifting centres of upwelling drove changes in planktic and benthic ecology, and were collectively instrumental in halting CO2 rise in the mid-deglaciation.

Description: The Antarctic Cold Reversal (ACR; 14.7 to 13 ka) phase of the last deglaciation saw a pause in the rise of atmospheric pCO2 and Antarctic temperature, contrasted with warming in the North. Mechanisms associated with interhemispheric heat transfer have been proposed to explain features of this event, but the response of marine biota and the carbon cycle are debated. The Southern Ocean is a key site of deep-water exchange with the atmosphere, hence deglacial changes in nutrient cycling, circulation, and productivity in this region may have global impact. Here we present a new perspective on the sequence of events in the deglacial Southern Ocean, that includes multi-faunal benthic assemblage (foraminifera and cold-water corals) and geochemical data (Ba/Ca, 14C, δ11B) from the Drake Passage. Our records feature anomalies during peak ACR conditions indicative of circulation, biogeochemistry, and regional ecosystem perturbations. Within this cold episode, peak abundances of thick-walled benthic foraminifera and cold-water corals are observed at shallow depths in the sub-Antarctic (~300 m), while coral populations at greater depths and further south diminished. Geochemical data indicate that habitat shifts were associated with enhanced primary productivity in the sub-Antarctic, a more stratified water column, and poorly oxygenated bottom water. These results are consistent with northward migration of primary production in response to Antarctic cooling and widespread biotic turnover across the Southern Ocean. We suggest that expanding sea ice, suppressed ventilation, and shifting centres of upwelling drove changes in planktic and benthic ecology, and were collectively instrumental in halting CO2 rise in the mid-deglaciation.

Description: The oceanic gateways of the Drake Passage and the Agulhas Current are critical locations for the inflow of intermediate-depth water masses to the Atlantic, which contribute to the shallow return flow that balances the export of deep water from the North Atlantic. The thermohaline properties of northward flowing intermediate water are ultimately determined by the inflow of water through oceanic gateways. Here, we focus on the less well-studied "Cold Water Route" through the Drake Passage. We present millennially-resolved bottom current flow speed and sea surface temperature records downstream of the Drake Passage spanning the last 25,000 yr. We find that prior to 15 ka, bottom current flow speeds at sites in the Drake Passage region were dissimilar and there was a marked anti-phasing between sea surface temperatures at sites upstream and downstream of the Drake Passage. After 14 ka, we observe a remarkable convergence of flow speeds coupled with a sea surface temperature phase change at sites upstream and downstream of Drake Passage. We interpret this convergence as evidence for a significant southward shift of the sub-Antarctic Front from a position north of Drake Passage. This southward shift increased the through-flow of water from the Pacific, likely reducing the density of Atlantic Intermediate Water. The timing of the southward shift in the sub-Antarctic Front is synchronous with a major re-invigoration of Atlantic Meridional Overturning Circulation, with which, we argue, it may be linked.

Description: The Antarctic Cold Reversal (ACR; 14.7 to 13 ka) phase of the last deglaciation saw a pause in the rise of atmospheric pCO2 and Antarctic temperature, contrasted with warming in the North. Mechanisms associated with interhemispheric heat transfer have been proposed to explain features of this event, but the response of marine biota and the carbon cycle are debated. The Southern Ocean is a key site of deep-water exchange with the atmosphere, hence deglacial changes in nutrient cycling, circulation, and productivity in this region may have global impact. Here we present a new perspective on the sequence of events in the deglacial Southern Ocean, that includes multi-faunal benthic assemblage (foraminifera and cold-water corals) and geochemical data (Ba/Ca, 14C, δ11B) from the Drake Passage. Our records feature anomalies during peak ACR conditions indicative of circulation, biogeochemistry, and regional ecosystem perturbations. Within this cold episode, peak abundances of thick-walled benthic foraminifera and cold-water corals are observed at shallow depths in the sub-Antarctic (~300 m), while coral populations at greater depths and further south diminished. Geochemical data indicate that habitat shifts were associated with enhanced primary productivity in the sub-Antarctic, a more stratified water column, and poorly oxygenated bottom water. These results are consistent with northward migration of primary production in response to Antarctic cooling and widespread biotic turnover across the Southern Ocean. We suggest that expanding sea ice, suppressed ventilation, and shifting centres of upwelling drove changes in planktic and benthic ecology, and were collectively instrumental in halting CO2 rise in the mid-deglaciation.

Description: Explanations of the glacial-interglacial variations in atmospheric pCO2 invoke a significant role for the deep ocean in the storage of CO2. Deep-ocean density stratification has been proposed as a mechanism to promote the storage of CO2 in the deep ocean during glacial times. A wealth of proxy data supports the presence of a "chemical divide" between intermediate and deep water in the glacial Atlantic Ocean, which indirectly points to an increase in deep-ocean density stratification. However, direct observational evidence of changes in the primary controls of ocean density stratification, i.e., temperature and salinity, remain scarce. Here, we use Mg/Ca-derived seawater temperature and salinity estimates determined from temperature-corrected d18O measurements on the benthic foraminifer Uvigerina spp. from deep and intermediate water-depth marine sediment cores to reconstruct the changes in density of sub-Antarctic South Atlantic water masses over the last deglaciation (i.e., 22-2 ka before present). We find that a major breakdown in the physical density stratification significantly lags the breakdown of the deep-intermediate chemical divide, as indicated by the chemical tracers of benthic foraminifer d13C and foraminifer/coral 14C. Our results indicate that chemical destratification likely resulted in the first rise in atmospheric pCO2, whereas the density destratification of the deep South Atlantic lags the second rise in atmospheric pCO2 during the late deglacial period. Our findings emphasize that the physical and chemical destratification of the ocean are not as tightly coupled as generally assumed.